This invention relates to a ballast for a high-pressure gas discharge lamp of a motor vehicle having: a primary pulsed, direct-voltage converter, including a converter transformer, a primary-side converter switch (switching transistor), and a secondary-side rectifier circuit; an H-bridge circuit and an ignition circuit downstream of the direct-voltage converter; a pulse-width modulator, whose output signal is applied to a control input of the primary-side converter switch; and a regulator whose control-signal value is applied to an input of the pulse-width modulator and determines the pulse width of an output signal generated by the pulse-width modulator.
German patent document (DE 41 26 865 A1) discloses a circuit of this type. German patent document (DE 41 26 865 A1) describes a ballast wherein, in an ignition-transition phase, a high-pressure gas discharge lamp is supplied in parallel from a direct-voltage converter and a booster circuit.
In this and similar ballasts, the problem is that a maximum open-circuit output voltage provided by the direct-voltage converter represents a constant value. In a ballast having a direct-voltage converter, this open-circuit output voltage and a maximum output power determine how reliably an ignition spark is transformed into a stable arc discharge upon starting the high-pressure gas discharge lamp. The maximum converter output power depends on the operating voltage (with a limited operating current), and decreases as the operating voltage decreases. The reliability of the transformation of the ignition spark into the stable arc thereby decreases as the converter power decreases. When the operating voltage is reduced, a reliable ignition of the high-pressure gas discharge lamp is no longer assured in every instance.
It is an object of this invention therefore to provide, in an uncomplicated and inexpensive manner, a ballast for a high-pressure gas discharge lamp of a motor vehicle, which provides a particularly reliable ignition of the high-pressure gas discharge lamp even if the ballast is supplied with a reduced operating voltage.